Pressure Volume Temperature System

ABSTRACT

A method and an apparatus for characterizing a fluid including a phase transition cell to receive the fluid, a piston to control fluid pressure, a pressure gauge to measure the fluid pressure and to provide information to control the piston, and connectors to connect the cell, piston, and gauge. The exterior volume of the phase transition cell, piston, gauge, and connectors is less than about 10 liters. A method and an apparatus to characterize a fluid including observing a fluid in an phase transition cell, measuring a pressure of the fluid, and adjusting a pressure control device in response to the measuring.

BACKGROUND

The oil and gas industry has developed various tools capable of determining formation fluid properties. For example, borehole fluid sampling and testing tools such as Schlumberger's Modular Formation Dynamics Testing (MDT) Tool can provide important information on the type and properties of reservoir fluids in addition to providing measurements of reservoir pressure, permeability, and mobility. These tools may perform measurements of the fluid properties downhole, using sensor modules on board the tools. These tools can also withdraw fluid samples from the reservoir that can be collected in bottles and brought to the surface for analysis. The collected samples are routinely sent to fluid properties laboratories for analysis of physical properties that include, among other things, oil viscosity, gas-oil ratio, mass density or API gravity, molecular composition, H₂S, asphaltenes, resins, and various other impurity concentrations.

The reservoir fluid may break phase in the reservoir itself during production. For example, one zone of the reservoir may contain oil with dissolved gas. During production, the reservoir pressure may drop to the extent that the bubble point pressure is reached, allowing gas to emerge from the oil, causing production concerns. Knowledge of this bubble point pressure may be helpful when designing production strategies

Characterizing a fluid in a laboratory utilizes an arsenal of devices, procedures, trained personnel, and laboratory space. Successfully characterizing a fluid in a wellbore uses methods, apparatus, and systems configured to perform similarly with less space and personal attention and to survive in conditions that quickly destroy traditional lab equipment. Identifying the undesired phase change properties of a fluid is especially useful when managing a hydrocarbon reservoir.

SUMMARY

Embodiments herein relate to a method and an apparatus for characterizing a fluid including a phase transition cell to receive the fluid, a piston to control fluid pressure as the fluid flows from the cell, a pressure gauge to measure the fluid pressure and to provide information to control the piston, and connectors to connect the cell, piston, and gauge. Embodiments herein relate to a method and an apparatus to characterize a fluid including observing a fluid in an phase transition cell, measuring a pressure of the fluid during pressurization or depressurization, and adjusting a pressure control device in response to the measuring.

FIGURES

FIG. 1 is a schematic of a drilling system according to embodiments herein.

FIG. 2 is a flow chart of one embodiment of a process according to embodiments herein.

FIG. 3 is schematic drawings of an embodiment of an experimental PVT apparatus, including a phase transition cell for saturation pressure detection with optical measurements, a microfluidic vibrating tube densitometer for density measurements, and a microfluidic vibrating wire viscometer for viscosity measurements for use downhole.

FIG. 4 is a sectional view of an o-ring, backup ring, metal flange, and electrically isolating backup ring according to embodiments herein.

FIG. 5 is a sectional view of a vibrating tube densitometer with integrated isolation components according to embodiments herein.

FIG. 6 is a sectional view of a sensor block containing modular sensors according to embodiments herein.

FIGS. 7A and 7B are embodiments of a breadboard ‘plug and play’ based microfluidic system allowing sensors to be replaced and exchanged according to embodiments herein.

FIG. 8 is a sectional schematic view of a fluid path for ‘direct connect’ sensor configuration. according to embodiments herein

FIGS. 9A and 9B are a sectional view of a fluidic connector and a schematic of a receiving element according to embodiments herein.

FIGS. 10A and 10B are sectional views of two embodiments of O-rings disposed on connectors according to embodiments herein.

FIG. 11 is a schematic diagram of alignment pins and holes to facilitate sensor and jumper attachments according to embodiments herein.

FIG. 12 is a plot of pressure and temperature as a function of time for test conditions according to embodiments herein.

FIGS. 13A, 13B, and 13C are schematic diagrams of ‘plug and play’ and ‘direct connect’ based microfluidic system fluid paths according to embodiments herein.

FIG. 14 is a schematic of an embodiment comprising a Peltier cooler according to embodiments herein.

FIG. 15 is a schematic drawing of an embodiment of an experimental PVT apparatus, including a phase transition cell for saturation pressure detection with optical measurements, a microfluidic vibrating tube densitometer for density measurements, and a microfluidic vibrating wire viscometer for viscosity measurements.

FIG. 16A is an embodiment of the optical signal density of the measured signal during depressurization when thermal nucleation was applied and not applied according to embodiments herein.

FIG. 16B shows density and viscosity measurements during depressurization shown in FIG. 15.

FIG. 17 is a phase diagram measured of a multi alkane sample with and without thermal nucleation of the sample fluid according to embodiments herein.

FIG. 18A is a plot of the difference of the saturation pressure measured with and without thermal nucleation as a function of temperature according to embodiments herein.

FIG. 18B is a plot of density as a function of pressure for low (23° C.) and high (125° C.) temperatures according to embodiments herein.

FIG. 19A is a plot of the optical densities as a function of pressure for saturation pressure measurements taken at temperatures ranging from 22.7° C. to 148.7° C. and plotted for each temperature according to embodiments herein. The temperatures correspond to the plots on a one to one basis.

FIG. 19B is a plot of a resulting phase diagram that includes data obtained with and without thermal nucleation according to embodiments herein. The temperatures correspond to the plots on a one to one basis.

FIG. 20A is a plot of the viscosity for each depressurization temperature according to embodiments herein. The temperatures correspond to the plots on a one to one basis.

FIG. 20B is a plot of the corresponding density of FIG. 19A. The temperatures correspond to the plots on a one to one basis.

FIG. 21 is a plot of the corresponding compressibility of FIG. 19A. The temperatures correspond to the plots on a one to one basis.

DESCRIPTION

At the outset, it should be noted that in the development of any such actual embodiment, numerous implementation-specific decisions may be made to achieve the developer's specific goals, such as compliance with system related and business related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time consuming but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. In addition, the composition used/disclosed herein can also comprise some components other than those cited. In the summary and detailed description, each numerical value should be read once as modified by the term “about” (unless already expressly so modified), and then read again as not so modified unless otherwise indicated in context. Also, in the summary and detailed description, it should be understood that a concentration range listed or described as being useful, suitable, or the like, is intended that any concentration within the range, including the end points, is to be considered as having been stated. For example, “a range of from 1 to 10” is to be read as indicating each possible number along the continuum between about 1 and about 10. Thus, even if specific data points within the range, or even no data points within the range, are explicitly identified or refer to a few specific points, it is to be understood that inventors appreciate and understand that any and all data points within the range are to be considered to have been specified, and that inventors possessed knowledge of the entire range and all points within the range.

The statements made herein merely provide information related to the present disclosure and may not constitute prior art, and may describe some embodiments.

Embodiments disclosed herein provide a means for measuring the temperature dependence of several fluid properties, including but not limited to, density, viscosity, and the bubble point. A pressure-volume-temperature (PVT) apparatus may be deployed in a downhole tool that could operate in an open or cased hole environment during a sampling job, but the PVT apparatus may also have applicability for production logging and surface applications. For downhole applications, the temperature of the PVT apparatus can be controlled to bring the sampled fluid to those temperatures that the fluid would be subjected to during production as the fluid was transported from reservoir to the surface.

As illustrated in FIG. 1, a drilling system 300 includes a bottom hole assembly 302 connected at the bottom end of a drill string 301 suspended within a wellbore 303. One or more other downhole tools may be located along the drill string 301 or along a wireline in the wellbore when the drill string 301 and bottom hole assembly 302 are removed from the well. Further, in accordance with one or more embodiments, a PVT device 160 may be contained within a downhole tool 305 which may be located along the drill string 301, on a wireline (not shown) or within a downhole tool (not shown). The PVT device 160 may be electrically connected to a component of a motor (not shown) or a battery (not shown) to receive energy therefrom. It should be understood that no limitation is intended by the arrangement of the drilling system, including the presence of absence of one or more components. As mentioned above, it is also envisioned that the drill string 301 may also be replaced by structures such as a wireline or any other apparatuses to convey the PVT device 160 into the wellbore, where the PVT device 160 is electrically connected to one or more tools located on the wireline.

Also shown in FIG. 1, near the bottom of the well, the pressure may be sufficiently high that the fluid is single-phase. At a given mid-point (the location of which may vary depending on well properties), the pressure may reach the bubble point when the fluid breaks phase, producing gaseous and liquid phases. While the fluid is transiting from the wellbore bottom to the surface, the temperature is monotonically decreasing, increasing the fluid viscosity.

Fluids that may be produced from the formation have their temperature changed as they are brought to the surface, and hence experience a dramatic change in the fluid properties, including but not limited to their viscosity. In order to accurately calculate the flow rate during production, an accurate knowledge of the viscosity as a function of depth is useful. Along with temperature dependence, the fluid pressure may drop below the bubble point while in transit. System disclosed herein may obtain a fluid sample from the formation and rapidly vary its temperature in order to simulate the fluid's passage through the oilwell during the production stage. In some embodiments, the PVT device 160 may store a sample extracted from the formation after measurements are performed. The PVT device 160 may be raised to a shallower depth and allow the sample within the PVT device 160 to come to equilibrium, after which additional measurements may be performed.

As an example, a description for measuring viscosity will be discussed, with a comparison of the amount of energy to change the sample temperature for both mesoscopic and microfluidic approaches. This would apply as well to a bubble point measurement where one is interested in the temperature dependence as well. The present embodiments may be compared to a conventional viscometer that is macroscopic in size and is directly immersed in the flow-line which has an inner diameter of approximately 5.5 mm. The total amount of fluid to fill the conventional sensors and the surrounding region volume is on the order of 10 milliliters, with an associated heat capacity of, assuming the specific heat of mineral oil, 1.7 Joules/(gram Kelvin), or a heat capacity of approximately 20 Joules/Kelvin. Hence, 20 Joules of energy are removed to reduce the temperature by one degree Kelvin. Furthermore, as the sensors are thermally connected to a large metallic assembly on the order of 1 kilogram (or more), in practice one would reduce the temperature of this assembly as well. Assuming a specific heat of 0.5 Joules/(gram Kelvin) for steel, one would have to remove 500 Joules of energy to reduce the temperature of the whole assembly by one degree. This approach using conventional technologies will be referred to as mesoscopic herein.

As a comparison, microfluidic environments of the present disclosure may use fluid volumes on the order of ten microliters, which corresponds to around 10 milligrams of liquid, which has a heat capacity of about 0.02 Joules/Kelvin (using the above numbers for the specific heat). In practice, one controls the temperature of the microfluidic chamber as well, which may have a mass on the order of 50 grams, and assuming this is fabricated from titanium, with a specific heat of 0.5 Joules/(gram Kelvin), it would use on the order of 25 Joules of energy to change the temperature by one degree. Note that this power usage for the microfluidic approach is 20 times smaller than for mesoscopic approach. Peltier (or thermoelectric) coolers reveals that models with dimensions with the proper scale exist and are specified to produce heat fluxes on the order of 1 Joule/second (1 watt), and one may quickly ramp up or down the temperature of such a device. Hence, a rapid ramping up or down of the temperature of a microfluidic-scale of fluidic volume and associated chamber is feasible.

FIG. 2 is a flow chart illustrating a process 200 for an embodiment of sampling fluid into a microfluidic system. A fluid may be sampled from a formation 201. In some embodiments, a small volume (on the order of tens of microliters) of fluid will be sampled, filtered, and passed into a microfluidic system. The system may be placed into a pressure compensation system where during the initial phase of its operation, the pressure is approximately 100 psi lower (or less) than the flowline of the tool in which it will be implemented. The microfluidic system may include microfluidic sensors to measure the density, viscosity or any other physical properties of the fluid. The microfluidic system may either be located downhole or at the surface. The microfluidic system may be such as that described in FIG. 3 or FIG. 15.

The fluid sample may then be introduced into microfluidic sensors. The pressure and temperature may be controlled precisely and rapidly so there is minimal thermal mass. The fluid sample may pass through a fluid path, such as those shown in FIGS. 6-11. The temperature and pressure controls act on the microfluidic sensors. The temperature may be controlled precisely and rapidly as there is minimal thermal mass, pressure may be controlled rapidly as pressure changes propagate at the speed of sound. For example, the pressure control may raise or lower the fluid pressure via motion of the piston thereby reducing or enlarging the volume available to the fluid. The temperature control can raise or lower the fluid temperature with a Peltier device or similar device, as well as raise or lower the temperature of the entire sensor, if desired. The microfluidic sensors may be used to measure density and viscosity as the temperature and pressure are controlled and monitored. The fluid may then be ejected 203 to the borehole. The measurement may then be repeated.

For downhole applications, this evaluation may be motivated by the fact that wellbore temperature changes substantially from the formation to the surface (FIG. 1). Fluids that are produced from the formation change their temperature accordingly and hence experience a dramatic change in their properties, including but not limited to their viscosity. In order to accurately calculate the flow rate during production one should accurately know the viscosity as a function of depth. This is further complicated by the fact that the fluid may drop below the bubble point while in transit. Hence, a system may be selected that can obtain a fluid sample from the formation and rapidly vary its temperature in order to simulate its passage through the wellbore during the production stage.

Generally, embodiments disclosed herein relate to collecting a fluid from a wellbore, a fracture in a formation, a body of water or oil or mixture of materials, or other void in a subterranean formation that is large enough from which to collect a sample. The fluid may contain solid particles such as sand, salt crystals, proppant, solid acids, solid or viscous hydrocarbon, viscosity modifiers, weighing agents, completions residue, or drilling debris. The fluid may contain water, salt water, hydrocarbons, drilling mud, emulsions, fracturing fluid, viscosifiers, surfactants, acids, bases, or dissolved gases such as natural gas, carbon dioxide, or nitrogen.

Systems for analyzing these fluids may be located in various locations or environments, including, but not limited to, tools for downhole use, permanent downhole installations, or any surface system that will undergo some combination of elevated pressures, temperatures, and/or shock and vibration. In some embodiments, temperatures may be as high as about 175° C. or about 250° C. with pressures as high as about 25,000 psi.

In general, energy added to a fluid at pressures near the bubble point to overcome the nucleation barrier associated with bubble production. Thus, energy may be added to a fluid thermally through the process of thermal nucleation. The quantity of bubbles produced at the thermodynamic bubble point via thermal nucleation is sufficiently small that their presence is detectable near the place of thermal nucleation in a phase transition cell and not in other components in the measurement system. However, upon further depressurization of the system, the supersaturation becomes large enough that bubble nucleation spontaneously occurs throughout the measurement system. In one or more embodiments, a fluid sample may be depressurized at a rate such that bubble detection may occur in a phase transition cell alone, or may be sufficiently high enough to be detected throughout the overall system.

During depressurization of a sample, the density, viscosity, optical transmission through the phase transition cell, and sample pressure may be simultaneously measured. Depressurization starts at a pressure above the saturation pressure and takes place with a constant change in system volume, a constant change in system pressure, or discreet pressure changes.

Collecting and analyzing a small sample with equipment with a small interior volume allows for precise control and rigorous observation when the equipment is appropriately tailored for measurement. At elevated temperatures and pressures, the equipment may also be configured for effective operation over a wide temperature range and at high pressures. Selecting a small size for the equipment is advantageous for rugged operation because the heat transfer and pressure control dynamics of a smaller volume of fluid are easier to control then those of large volumes of liquids. That is, a system with a small exterior volume may be selected for use in a modular oil field services device for use within a wellbore. A small total interior volume can also allow cleaning and sample exchange to occur more quickly than in systems with larger volumes, larger surface areas, and larger amounts of dead spaces. Cleaning and sample exchange are processes that may influence the reliability of the phase transition cell. That is, the smaller volume uses less fluid for observation, but also can provide results that are more likely to be accurate.

The minimum production pressure of the reservoir may be determined by measuring the saturation pressure of a representative reservoir fluid sample at the reservoir temperature. In a surface measurement, the reservoir phase envelope may be obtained by measuring the saturation pressure (bubble point or dewpoint pressures) of the sample using a traditional pressure-volume-temperature (PVT) view cell over a range of temperatures. Saturation pressure can be either the bubble or dewpoint of the fluid, depending upon the fluid type. At each temperature, the pressure of a reservoir sample is lowered while the sample is agitated with a mixer. This is done in a view cell until bubbles or condensate droplets are optically observed and is known as a Constant Composition Expansion (CCE). The PVT view cell volume is on the order of tens to hundreds of milliliters, thus using a large volume of reservoir sample to be collected for analysis. This sample can be consumed or altered during PVT measurements. A similar volume may be used for each additional measurement, such as density and viscosity, in a surface laboratory. Thus, the small volume of fluid used by microfluidic sensors of the present disclosure (approximately 1 milliliter total for measurements described herein) to make measurements may be highly advantageous.

In one or more embodiments, an optical phase transition cell may be included in a microfluidic PVT tool. It may be positioned in the fluid path line to subject the fluid to optical interrogation to determine the phase change properties and its optical properties. U.S. patent application Ser. No. 13/403,989, filed on Feb. 24, 2012 and United States Patent Application Publication Number 2010/0265492, published on Oct. 21, 2010 describe embodiments of a phase transition cell and its operation. Both of these applications are incorporated by reference herein. The pressure-volume-temperature phase transition cell may contain as little as 300 μl of fluid. The phase transition cell detects the dew point or bubble point phase change to identify the saturation pressure while simultaneously nucleating the minority phase.

The phase transition cell may provide thermal nucleation which facilitates an accurate saturation pressure measurement with a rapid depressurization rate of from about 10 to about 200 psi/second. As such, a saturation pressure measurement (including depressurization from reservoir pressure to saturation pressure) may take place in less than 10 minutes, as compared to the saturation pressure measurement via standard techniques in a surface laboratory, wherein the same measurement may take several hours.

Some embodiments may include a view cell to measure the reservoir asphaltene onset pressure (AOP) as well as the saturation pressures. Hence, the phase transition cell becomes a configuration to facilitate the measurement of many types of phase transitions during a CCE.

In one or more embodiments, a densitometer, a viscometer, a pressure gauge and/or a method to control the sample pressure with a phase transition cell may be integrated so that most sensors and control elements operate simultaneously to fully characterize a live fluid's saturation pressure. In some embodiments, each individual sensor itself has an internal volume of no more than 20 microliters (approximately 2 drops of liquid) and by connecting each in series, the total volume (500 microliters) to charge the system with live oil before each measurement may be minimized. In some embodiments, the fluid has a total fluid volume of about 1.0 mL or less. In other embodiments, the fluid has a total fluid volume of about 0.5 mL or less.

This configuration is substantially different than a traditional Pressure-Volume-Temperature (PVT) apparatus, but provides similar information while reducing the amount of fluid consumed for measurement. FIG. 3 is a schematic of one embodiment of a PVT apparatus for use downhole. In some embodiments, the PVT apparatus may be included into another measurement tool or may be standalone on a drill string or wire line. Although FIG. 3 includes a phase transition cell 140 for saturation pressure detection with optical measurements, a microfluidic vibrating tube densitometer 141 for density measurements, and a microfluidic vibrating wire viscometer 142 for viscosity measurements, some embodiments may include density or viscosity sensors to measure fluid properties as the cell depressurizes or pressurizes, while other embodiments may benefit from the phase transition cell working with no additional sensors at all. Compressibility measurements may also occur in some embodiments. The compressibility may be measured from the derivative of volume with respect to pressure with knowledge of the system 160 volume.

The control of the pressure within the system may use a pressure control device 143 or an alternate pressure control device, such as one based on a sapphire piston. In such an embodiment, the control of the pressure in the system may be adjusted by moving the piston to change the volume inside the piston housing 145 (partially shown) and, thus, the sample volume. The system's small dead volume (less than 0.5 mL) facilitates pressure control and sample exchange. In some embodiments, the depressurization or pressurization rate of the fluid is less than 200 psi/second. In some embodiments, the fluid is circulated through the system at a volumetric rate of no more than 1 ml/sec. Teflon, alumina, ceramic, zirconia or metal with seals may be selected for some components for various embodiments of the pressure control device. Smooth hard surfaces may be used to minimize friction of the moving piston and both energized and dynamic seals may be used.

The sample fluid is in pressure communication with the pressure gauge 144. The pressure gauge 144 can measure small pressure changes such as 2 to 3 psig. The gauge utilizes small sample volume for its external housing and also has low dead volume of less than about 1 mL. Some embodiments may have a dead volume of less than 0.5 mL or less than 0.05 mL.

The phase transition cell 140 includes a 2 mm long flowline constrained by two sapphire windows or lenses, United States Patent Application Publication Number 2010/0265492 provides additional details and is incorporated by reference herein. Light in the optical path between the two windows or lenses is highly sensitive to the presence of fluid interfaces, such as that associated with bubbles in a liquid (produced at bubble point) or liquid droplets in a gas (produced at dew point). An 80 percent Nickel, 20 percent Chromium (NICHROME80™) wire of diameter 100 microns or less is installed orthogonal to the flow path in the phase transition cell to thermally agitate the fluid to overcome the nucleation barrier, Some embodiments may use a wire comprising platinum, tungsten, iridium or a platinum-iridium alloy. A high current pulse (c.a. 5 amperes) of duration 5 microseconds quickly heats the fluid surrounding the wire by about 25° C. As the heat dissipates (in about 0.1 s) and the local temperature returns to that of the system, the bubbles formed in a liquid sample either collapse or remain stable, according to whether the system is above the saturation pressure or, inside the two-phase region, respectively. The mechanisms of the nucleation process and its operability on both sides of the cricondenbar are described in U.S. patent application Ser. No. 13/403,989, filed on Feb. 24, 2012 and U.S. patent application Ser. No. 13/800,896, filed on Mar. 13, 2013. Both of these references are incorporated by reference herein in their entireties.

As mentioned above, the tool of the present disclosure may include a densitometer 141 to measure fluid density which may be used to calculate compressibility. The fluid compressibility, k, can be calculated by precisely measuring the fluid density while varying the pressure. The compressibility can be defined as the relative change in fluid density with the change in pressure as in the following equation:

$\begin{matrix} {{k\lbrack\rho\rbrack} = {\frac{1}{\rho}\frac{\partial\rho}{\partial P}}} & (1) \end{matrix}$

In practice, the noise introduced by taking a derivative can be minimized by first smoothing and then fitting a local second order polynomial to the reciprocal of the density data. Due to the curvature of the data with pressure, the fit is more accurate when applied to the reciprocal of the density as compared to the fit directly on the density itself. For each pressure, the subset of the density data includes 31 densities that are centered on the pressure of interest. In theory, this corresponds to a pressure range of thousands of psi, but in practice this range covers a few hundred psi. The local fit can then be described as fitting the inverse density to a second order polynomial:

$\begin{matrix} {\frac{1}{\rho \lbrack P\rbrack} = {A + {BP} + {CP}^{2}}} & (2) \end{matrix}$

Determination of the local coefficients A,B,C, allows one to analytically calculate their derivative and then plug into the above compressibility equation as

$\begin{matrix} {{k\lbrack P\rbrack} = {{- \frac{\left( {B + {2\; {CP}}} \right)}{A + {BP} + {CP}^{2}}} = {{\rho \lbrack P\rbrack}\left( {B + {2{CP}}} \right)}}} & (3) \end{matrix}$

In practice, this smoothes the compressibility measurement while not introducing a strong bias. It has the further advantage of being model-independent, thereby being applicable regardless of the fluid's proximity to the critical point.

FIG. 3 provides a schematic view of one embodiment of the phase transition cell in combination with other elements. The components may be configured to work together or individually to observe a fluid sample. The devices present in the figure may be used in one system. They may be used individually in one system or a combination of some of them may be used. Each of the individual components may be in contact with the control system (not shown). The control system is in contact with the components and with an operator who is using a computer at the surface of the formation or Other location. The control system is electronic and may control the mechanics of the components. Throughout the elements, several temperature sensors may be embedded in devices or tubing connections to observe the temperature of the fluid.

In one embodiment, the fluid is collected through a membrane 146 as described in U.S. Pat. No. 7,575,681, issued on Aug. 18, 2009, and U.S. Pat. No. 8,262,909, issued on Sep. 11, 2012. Both of these references are incorporated by reference herein. The membrane 146 is housed in a frame configured for supporting the membrane even during exposure to harsh environments and for cleaning activities including backflush backflushing to remove particulate buildup from the membrane. In some embodiments, the membrane 146 prevents particles with a dimension of 10 micron or greater to flow through the membrane. In some embodiments, the membrane is hydrophobic. As pictured, the fluid is flowed through the membrane 146 as in a cross-flow. In some embodiments, fluid is flowed across the membrane as in dead-end filtration.

Next, the fluid collects behind the membrane 146 and flows through tubing on to an entry valve 147. The entry valve 147 is a needle valve or ball valve or other valve that is selected for its volume and fluid flow properties. The entry valve 147 features a small dead volume and precise open and close control. The entry valve 147 is controlled to allow or prevent a specific fluid flow to the phase transition cell and/or to allow backflushing of the membrane 146. The valve 147 may be closed completely in some operations. It is selected to be modular and low cost for maintenance and repair.

Then, the fluid flows through the phase transition cell 140 as described above. From the phase transition cell, fluid flows through a densitometer 141. The small volume of the fluid flowing through the densitometer 141 utilizes a carefully selected cross sectional area and fluid flow path. The risk of deposition and/or flocculation of asphaltenes and other highly viscous or readily precipitating material also influences the design. One example of such a densitometer is described in U.S. Patent Publication No. 2010/0268469 published on Oct. 21, 2010, which is incorporated by reference, in its entirety, herein.

Then, the fluid flows through a viscometer 142. Like the densitometer 141, the viscometer 142 contains a small volume of fluid and utilizes a carefully selected cross sectional area and fluid flow path. A similar risk of surface contamination exists and thoughtful design elements and considerations are considered. One example of such a viscometer is described in U.S. patent application Ser. No. 13/353,339, filed on Jan. 19, 2012, which is incorporated by reference, in its entirety, herein.

The fluid enters the pressure control device 143 such as a sapphire based piston and then exerts a pressure on the pressure gauge 144. The pressure gauge can measure small pressure changes with a precision better than 0.1 psi and an accuracy of 2 to 3 psig under downhole conditions. The gauge has low volume for its external housing and also has low dead volume of about 0.5 mL or less.

Next, the fluid flows on to an exit valve 148. Like the entry valve 147, the exit valve 148 is a needle valve or other valve that is selected for its volume and fluid flow properties. The exit valve 148 features a small dead volume and precise control. The exit valve 148 is controlled to allow or prevent a specific fluid flow to a back pressure regulator 149. In some embodiments, a back pressure regulator is not included. The valve 148 may be closed completely in some operations. It is selected to be modular and low cost for maintenance and repair. Some embodiments may include a bypass flow line 151 with a pressure gauge 152 and pressure control device 153. The fluid may be sent downhole through flow line 150. Embodiments could be implemented without a back pressure regulator and simply use the differential pressure created by the piston to induce fluid to flow into the microfluidic system through the membrane 146 in a dead-end filtration configuration.

In another embodiment, the membrane 146 is a cross-flow configuration. The piston pumps fluid through the membrane 146, the entry valve 147, and the exit valve 148. The valve configuration for pumping into the system is the entry valve 147 open and exit valve 148 closed, and the configuration for pumping out of the system (discharging used fluid) is the entry valve closed 147 and the exit valve open 148.

Some embodiments may have a phase transition cell, piston, and pressure gauge that have a combined external volume 154. This external volume 154 may be about 10 liters or less. The external volume 154 may be about 2.5 gallons or less. The external volume 154 may be configured to fit into a wellbore, a downhole oilfield tool or a formation evaluation tester.

System performance may depend on the arrangement of sensors in the flow path. Fluid becomes progressively more contaminated the further downstream from the sample inlet a sensor is, due to limitations of flushability of the fluidic connectors, sensors, and components. Therefore, the sensors that may be most sensitive to contamination or have the highest levels of accuracy should be put as near the inlet as possible. In the configuration shown in FIG. 3, the bubble point measurement is both sensitive to contamination and has high accuracy, so it is placed first. The densitometer is placed second, as this is a relatively high precision measurement but has lower sensitivity to contamination than the bubble point. The viscometer is located third, as the desired precision is low compared to either the densitometer or bubble point. The pressure gauge and piston which may be insensitive to contamination can be placed last. The arrangement of the sensors in FIG. 3 is one of many embodiments which can be implemented.

Some embodiments may have the apparatus including the phase transition cell 140, piston 143, and pressure gauge 144 that have a combined external volume. This external volume may be about 10 liters or less. The external volume may be about 2.5 gallons or less. The external volume may be configured to fit into a wellbore.

In some embodiments, the components (such as the phase transition cell 140, viscometer 143 and densitometer 144) may be connected by a device, such as a microfluidic union (shown in FIG. 4 in greater detail), that allows a hydraulic connection to be made between two metal tubes of such small inner diameter, of order 1 millimeter or less, that they could be described as capillaries, while maintaining electrical isolation between the two metal tubes. The microfluidic union may be described as an electrically isolating hydraulic connector. The device prevents random electrical noise, ever present in metal tubes connected to an electrically enabled instrument, to bias or prevent suitable operation of microsensors connected to this tubing. In particular, a microfluidic densitometer 144 or a microfluidic coriolis force meter (to measure mass flow rate, not shown) may benefit from this device. The device operates over a wide range of temperature (up to about 200° C.) and pressure (about 30,000 psi) while adding a negligible amount of dead or non-flushed volume and acts as a hydraulic union. The device is operable under a large amount of shock and vibration, as often encountered with logging while drilling (LWD) tools.

FIG. 4 is a sectional view of microfluidic junction 800. Junction 800 includes an o-ring 803, a backup ring 805, a metal flange 804 (which may be welded or brazed to tubing 801), and an electrically isolating backup ring 806 to connect first tubing 801 and second tubing 810. The tubing 801 may have a diameter 802 of 1 mm. The tubing 801 may be made of stainless steel, Hastelloy, medical grade tubing, etc. Tubing 801 may have a terminal end adjacent o-ring 803. Behind o-ring 803 are backup ring 805, metal flange 804 and electrically isolating backup ring 806 through which tubing 801 also extends. The backup ring 805 and isolating backup rings 806 may be made of PEEK, other polymers, ceramics, composite materials, etc. Some embodiments may use a flange 804 comprising steel, polymer, or other material. Some embodiments may also comprise a gland 807 and electrically isolating sleeve 808 through which tubing 801 also extends. The terminal end of tubing 801 (and o-ring 803, backup ring 805, metal flange 804, isolating backup ring 806 and gland 807) may be received by a union 809, also retained as a support device. Electrically isolated from the tubing 801, but in hydraulic connection is tubing 810. While tubing 810 may be a second tubing as described above, it is also within the scope of the present disclosure that tubing 810 may be in hydraulic connection with but electrically isolated from a third tubing using a junction similar to that illustrated in FIG. 4. The components support device or union 809 to connect a second capillary tube by laser welding or metal seal or other means.

The device of FIG. 4 functions to fluidically and hydraulically connect two metal tubes (801 and 810) while maintaining electrical isolation. A flange 804 may be welded or brazed onto the tube 801. An o-ring 803 and backup ring 805 may be stacked onto the tube 801. On the other side of the flange 804, an electrically isolating ring 806, made of a plastic or ceramic or other electrically insulating material or components, may be installed by sliding down the tube. Next, an electrically isolating sleeve 808 may be slid in place, and finally a gland 807. The gland 807 backs the stack of components under application of internal pressure. By proper choice of the dimensions of the flange 804 and the block 809 that the gland 807 screws into, the tubes 801 and 810 are electrically isolated from one another. While not explicitly shown, the tube 810 may be brazed or welded hydraulically to make a pressure tight seal to 809. “Gland” herein means a type of nut but with the threads on the outside instead of the inside and with a hole in the center (see FIG. 4). This type of geometry where one tube is inserted into a block or pressure housing is referred to as a “stabber” connection in oilfield parlance, as is often the case, sealing is achieved by a combination of an o-ring and backup ring. Stabbers do not generally create electrical isolation, but by careful choice of the flange dimensions and associated components electrical connectivity can be removed. A similar seal is made here (FIG. 4). The second capillary tube attached to the union may be sealed to the union by conventional means of a metal seal, a welded seal, or one of several other methods (see FIG. 4).

In some embodiments, an electrically isolating ring 806, comprising one of many electrically isolating materials, including but not limited to plastic such as poly ether ketone, mica, ceramics including silicon nitride or aluminum oxide, may be placed under compression to electrically isolate the gland from the flange. In some embodiments, the Outer Diameter (OD) 802 of the flange may be slightly smaller than that of the housing 804. A thin PEEK sleeve 808 may be placed between the capillary tube 801 and the gland 807. The gland 807 may be optional if the tubing 801 is sufficiently rigid to withstand a compressive force pushing on the other end of the first capillary tube 801 to “stab” it into the microfluidic union.

FIG. 5 is a view of a vibrating tube densitometer 1000 with integrated isolation components 1001. A thin tube is the vibrating element 1002 of the vibrating tube densitometer 1000.

Some embodiments feature implementation of microfluidic sensors in an actual downhole tool in different ways, including a modular ‘plug and play’ system which allows sensors to be easily replaced, moved, and exchanged with each other, as shown in FIG. 6. In such an implementation, the sample may flow from one sensor to the next, traveling through channels which connect the different sensing modules. The connections discussed herein (and shown in FIGS. 4, 9 and 10) serve as the basis for a system that operates at high pressure and high temperature (HPHT), high shock and vibration conditions downhole and/or on the surface.

As shown in FIG. 6, some embodiments provide multiple flowlines 1100, 1101 of the tools running through a sensor platform 1102 not as a single piece of tubing, but rather as a series of connections that connect different modular sensing components. Different sensing ‘blocks’ could be stacked in various configurations, tailored for the specifications of the system. FIG. 6 illustrates attaching together interchangeable sensor blocks 1103, 1104, 1105, 1106, 1107.

Each of these sensor ‘blocks’ may house multiple sensors and may operate as a stand-alone modular system. To make connections between these smaller systems or within each of these smaller systems, different low dead-volume connections 1108 may be selected.

Products able to operate at HPHT, under high shock and vibration conditions are commercially available from companies such as High Pressure Equipment™ and Swagelok™ These connections 1108 are metal-metal seals. The high pressure of a fluid is held inside the connection by the formation of a seal between a metal tube and a metal fitting. Torque applied to a gland holds this metal-metal seal in place. Generally, these metal pieces can be reused for a limited number of times before being replaced. In some embodiments, a polymeric o-ring may be utilized to form the seal which retains high pressure fluid. Force from clamping together the two parts being fluidically connected keeps the o-ring in place. The use of a polymer o-ring as the pressure retaining seal decreases the likelihood that a seal will fail after repeated use. An o-ring can be easily replaced, whereas replacing a metal seal could involve replacement/resizing of metal tubing as well as the metal fitting.

There are many configuration options for a modular ‘plug and play’ system for microfluidic components operating in downhole conditions. Such a system allows various sensors to be replaced without disturbing other sensors in the platform, easing the exchange of sensor functionally and replacement of faulty sensors. This module operates in series with other downhole non-microfluidic or microfluidic sensing blocks or as a stand-alone unit.

As shown in FIGS. 7 a and 7 b, in one arrangement, the fluid flows into a given sensor 1200 in one direction and flows out of the sensor in a different direction. The exit flow could be in any direction including, but not limited to, 180 degrees from the entrance flow. The entrance ports 1201 and exit ports 1202 of various sensors could attach to a common ‘breadboard’ chassis 1203. This breadboard 1203 features internal fluidic paths 1204 to route sample from one sensor to the next. The connections for the sensors could be located on both sides of the breadboard (FIG. 7 a) or one side (FIG. 7 b). If on one side, the breadboard would feature internal fluidic channels that, like the sensors, allow fluid to flow in one direction and out in a different direction. If on both sides, straight channels through the breadboard allow fluid to flow one sensor to the next. In other embodiments, the channels may not be straight.

This breadboard is a chassis for the larger tool that the microfluidic components fit into, or it is a stand-alone unit. In either embodiment, fluidic connections not made by a sensor or the breadboard are done with a microfluidic ‘jumper’. This jumper would serve as a simple fluidic connection to enable the sample to flow from one sensor to the next sensor or the breadboard.

Sensors are securely attached to the breadboard, locking them into place and forming the seal with the connectors to keep the sample inside the microfluidic path. This arrangement—one where the connectors are inserted into a common fluidic breadboard—allows a sensor to be moved and replaced without disturbing other sensors already installed in the system.

As shown in FIG. 8, in a ‘direct connect’ arrangement, the fluid either flows in and out of a sensor (or jumper) in the same direction or passes through from one end to the other. ‘Direct connect’ allows sensors to be replaced and exchanged. Sensors 1301, 1302, 1303 may be directly connected to one another—fluid would pass from one sensor or jumper to the next, without traveling through a breadboard. Sensors 1301, 1302, 1303 could be arranged in any order. This configuration would offer lower overall system volume then the ‘plug and play’ arrangement, but not the ease of replacing a sensor without disturbing the other sensors in the system.

A microfluidic ‘plug and play’ system for downhole use (i.e. HPHT, shock and vibration conditions) depends on the use of a microfluidic connection that allows components to be easily removed and replaced by other components. Such a connector, described below and shown in FIGS. 9A and 9B, has been developed and is suitable for downhole use.

The apparatus disclosed is a metal tube that has been cut to a specific length and machined to have at least three outer grooves running around the outer diameter. In some embodiments, more or less grooves may be used. O-rings rest in outer two of these grooves, and are retained in place with a metal ‘lips’ at the end of piece. This piece is a connector 1401 which allows fluid to flow from one device to another: the inner diameter 1402 of the connector may be hundreds of microns in diameter. Each device (bread board and/or sensor and/or jumper) features receiving holes 1403 for the connector which connect to a fluidic path 1404 that allows fluid to be transported through the device.

As shown in FIGS. 10 a and 10 b, polymeric o-rings 1501, 1502, 1503, and 1504 serve as the sealing mechanism to retain high pressure fluid inside the connectors. There are numerous configurations of o-rings that may be selected. FIGS. 10A and 10B provide two different configurations. In one, two polymeric (Viton™) o-rings 1501 and one PEEK o-ring 1502 are placed in grooves, and in the other one Viton™ o-ring 1503 and one PEEK o-ring 1504 are used. These are two examples of o-ring configurations and materials and there are more possibilities.

In the final system assembly, one connector 1401 is inserted into two receiving holes: these receiving holes could be in sensors, the breadboard or microfluidic jumpers. The receiving holes and connectors are designed to minimize the amount of ‘dead-volume’ fluid of the connection. The dead volume is the amount of fluid that exists at the connector/hole interface but not inside the flow channel of the connector. For example, the connector embodiment shown in FIGS. 10A and 10B with one Viton™ o-ring at each end, the fluidic dead volume (fluid not inside the connector) is 0.0006422 mL.

To facilitate the placement of a connector 1605 into a receiving hole 1604, alignment pins and holes, such as those shown in FIG. 11, can be used. These pins 1601 and holes 1602 help to guide the connector 1605 into the receiving hole 1604, preventing bending of the connector. An external alignment system, which may include guiding rails that can be removed once the fluidic connection is in place, can also be used, in addition or in combination with the alignment pins and holes.

Both the connectors shown in FIGS. 10A and 10B have been shown to hold high pressure at elevated temperature. Both types of connectors may be placed in an oven at 150° C., while the internal pressure of the sample inside raised and lowered in increasingly large steps. Ultimately, both configurations held 20,000 psi of pressure at 150° C. Test conditions are shown in FIG. 12.

Pressure retention while undergoing shock and vibration is also a design consideration. A connector with two Viton™ o-rings and one PEEK o-ring (as seen in FIG. 10) has also been shown to hold 20,000 psi of pressure while undergoing 100,000 shocks at 500 G (at room temperature). Table 1 summarizes the number of shocks and shock levels applied to the connectors. X and Y-axis refer to the orientation of the connectors with respect to the moving arm of the shock machine. In the orientations test the fluidic path of the connector was perpendicular to the arm of the shock machine: the X and Y-axis results of this test are interchangeable, since the connector is rotationally symmetric about its fluidic path.

TABLE 1 Number Fluid Pressure Axis of Shocks Shock Level Inside Connector X 3,000 500 G 20,000 psi Y 97,000 500 G 20,000 psi

The HPHT, shock and vibration resistant connectors described above can be integrated into modular microfluidic systems, as shown in FIGS. 13A, 13B, and 13C. In FIG. 13A, connectors 1704 allow fluid to move from the breadboard 1702 to sensors 1701 via fluidic paths 1703. The fluid path in the jumpers 1706 and the breadboard 1705 also enable fluid to move between sensors. Alignment pins 1707 help facilitate sensor installation. Alignment pins 1711 are also found in the one-sided chassis scheme shown in FIG. 13B. Fluid moves to and from sensor 1708 via connectors 1710 and fluid paths 1712 and 1709. 1712 is the fluid path internal to the chassis 1713. In the ‘direct connect’ configuration shown in FIG. 13C, sensors 1750 are fluidically connected via connectors 1753 and fluid paths 1751. Alignment pins 1752 can be used. Here, the function and placement of the connectors in the modular system is illustrated. When utilized as shown, these connectors enable low dead-volume ‘plug and play’ and ‘direct connect’ microfluidic apparatuses. The use of different connectors, such as metal-metal seals, would increase the volume of the system: additional tubing length between the sensors would have to be incorporated to leave physical access to the connectors.

In some embodiments, the connectors have a total internal volume of less than 1 mL. Some embodiments utilize connectors that form a fluid connection that survives a pressure of about 15,000 psi or more. Some embodiments include connectors that form a fluid connection that operates at a temperature of about 175° C. Some embodiments use connectors that include tubing. Sometimes, the tubing has an internal diameter of about 1.0 mm or less. Some embodiments benefit from tubing that has an internal diameter of 0.25 mm or less.

In some embodiments, the measurement of the fluid properties at the temperature and pressure of the formation alone does not suffice to predict the behavior of reservoir fluids because the temperature will change dramatically as it is pumped from the formation to the surface. Correlations are often inadequate as predictive indicators of such behavior. In some embodiments, the thermal mass of a small volume allows the temperature to be rapidly and accurately controlled such that the transport properties, including density and viscosity, can be then be measured as a function of temperature in a matter of minutes. This, when combined with a means of controlling the pressure of the sample, allows one to characterize the properties of the live fluid so that its behavior is understood in a predictive manner during its transit from the formation to the surface.

FIG. 14 is a Peltier cooler 1900 designated with the cool side towards a microfluidic sensor 1901 and the hot side (the side by which the Peltier cooler radiates heat so as to effectively cool the cold side) towards the tool housing. Referring to FIG. 14, the live downhole fluid will be introduced through the two microfluidic connections 1902 a and 1902 b on the top and the two upper valves 1903 a and 1903 b closed. Several measurements, of which viscosity, density, or bubble point could be one, will be made with the microfluidic sensors 1901. At this point, the system allows an operator to vary the pressure with the pressure compensation system 1904 and the temperature with the Peltier device 1905 while maintaining the same fluid sample in the microfluidic channels. If the pressure and temperature dependence of the well is known as a function of depth, an identical set of temperature-pressure points may be created in the microfluidic device so as to interrogate the fluid properties in a way that mimicked the fluid's transit to the surface.

As a further elaboration, after dropping the pressure below the bubble point one could separate the gaseous phase from the liquid phase so that a viscosity measurement could be exclusively performed on the liquid portion. By designing the microfluidic channels to be oil wet, a continuous oil stream could be siphoned off. The viscosity of this liquid stream is then measured. The viscosity measurement is then combined with temperature control to provide the viscosity of the liquid portion of the live fluid when the pressure is dropped below the bubble point.

Further details of using the PVT apparatus in conjunction with a wellbore tool and methods for implementing the PVT apparatus are described in U.S. patent application Ser. No. 13/829,710, entitled “Method to Perform Rapid Formation Fluid Analysis” and filed on Mar. 14, 2013.

EXPERIMENTAL RESULTS

A PVT apparatus as shown in FIG. 15 was used to investigate a multi-alkane sample as listed in Table 2.

TABLE 2 Component Mole percentage Methane 47 Ethane 24 n-Pentane 14 n-Hexane 10 n-Heptane 5

FIG. 15 is a schematic of a PVT apparatus 100. The components in the region indicated by dashed line 101 may be in a temperature-controlled oven or in a downhole oilfield tool. Within the oven 101, the PVT apparatus 100 may include a filter 105, valves 103 and 104 to isolate the sample, a phase transition cell 106, a densitometer 107, a viscometer 108, a pressure gauge 109, and a floating piston 114. Outside the oven 101, the PVT apparatus 100 may include a pressure gauge 110, a Single Phase Sample Bottle (SSB) 111, and at least two valves 112 to direct pressure control from a pump 113 between the SSB 111 and the floating piston 114.

The PVT apparatus 100 includes a single phase sample bottle (SSB) 111. The SSB 111 may be of the type produced by Schlumberger-Oilphase, Aberdeen. A pump 113 may be used to pressurize the SSB 111. The pump 113 may be an Isco 65D syringe pump filled with water (Teledyne Isco). The SSB 111 may include a floating piston 114 for maintaining pressure on the sample while providing fluidic isolation from the source of the pressure, i.e., pump 113.

A live fluid was stored in the Single-Phase Sample Bottle 111. The SSB 111 may be one such as that manufactured by Schlumberger-Oilphase, Aberdeen. The SSB 111 was pressurized with an pump 113 filled with water. The pump 113 may be an Isco 65D syringe pump (Teledyne Isco). The SSB 111 includes a floating piston 114 for maintaining pressure on the sample while providing fluidic isolation from the source of pressure, which in this case was water pressurized by pump 113. Tubing of Outer Diameter (OD) 1/16″ and Inner Diameter (ID) 0.020″ was used wherever possible in the experimental apparatus as a standard so as to reduce the system volume. A pressure gauge 109 with a customized low dead-volume fitting (7 microliters) was employed inside of the oven to measure the pressure of the sample during depressurization. The pressure gauge 109 may be a Kistler pressure gauge. Calibration of the pressure gauge 109 was performed against a pressure gauge 110 (outside oven at ambient temperature) of higher accuracy at each temperature before and after each experiment since the calibration was found to drift substantially upon a change in the temperature. The pressure gauge 110 may be a Quartzdyne pressure gauge.

The PVT system may initially be charged with a fluid such as hydraulic oil or an alkane mixture at ambient pressure and then pressurized with fluid hydraulically connected to the waste SSB. During an experiment the live oil was charged into the PVT system from the sample cylinder and discharged afterwards into a waste cylinder (which may be a SSB), thereby maintaining the sample pressure far above the saturation pressure so as not to break phase. Both cylinders were stored at ambient temperature outside of the oven, but the sample fluid was quickly heated to the oven temperature due to its low thermal mass (about 300 microliters charged into the system per measurement). A new aliquot of live oil was charged into the system for each depressurization experiment. The valves 112 on the two cylinders were closed after charging the system and the sample was isolated between valves 103 and 104. The valves 103 and 104 may be AF1 needle valves (High Pressure Equipment Company, HIP) which were located inside of the oven (FIG. 15), thereby maintaining the sample at uniform temperature. The MS series microreactor 115 from HIP may be used to controllably depressurize the isolated sample. The microreactor 115 includes a small floating piston 114 where pressure was controlled with an Isco pump. The microreactor 115 was hydraulically similar to the sample cylinder but may have a maximum volume of about 10 mL.

The fluid may be collected in the SSB 111 and flow through tubing via an entry valve 112 a. The entry valve 112 a may be a needle valve or other valve that is selected for its volume and fluid flow properties. The entry valve 112 a features a small dead volume and precise open and close control. The entry valve 112 a may be controlled to allow or to prevent a specific fluid flow to the phase transition cell and/or to allow backflushing of the filter 105. The valve 112 a may be closed completely in some operations. The valve 112 a may be selected to be modular and low cost for maintenance and repair.

Next, the fluid collects behind the filter 105 and flows through tubing to an entry valve 103. The entry valve 103 may be a needle valve, ball valve or other valve that is selected for its volume and fluid flow properties. The entry valve 103 features a small dead volume and precise opening and closing control. The entry valve 103 is controlled to allow or prevent a specific fluid flow to the phase transition cell and/or to allow backflushing of the filter 105. The valve 103 may be closed completely in some operations. The entry valve 103 may be selected to be modular and low cost for maintenance and repair. The valves 103 and 104 are configured such that the pressure experienced by both the pressure gauge 110 (outside oven) and pressure gauge 107 (inside oven) could be uniformly varied from about 1000 to about 8000 psi, thereby performing an in-situ calibration of the pressure gauge 107 at the temperature of the oven, with or without the use of the micropiston.

The PVT apparatus 100 may include a first valve (V1) 103 and a second valve (V2) 104. Valves 103 and 104 may be located in the oven with their valve handles situated outside such that they could be operated without opening the oven door and altering the temperature. In other embodiments, they may be controlled by a motor and associated electronics such that operation may be effected remotely. In some embodiments, the first valve 103 and second valve 104 may be AF1 valves (High Pressure Equipment Company, HIP). In other embodiments, the first valve 103 and second valve 104 may be a needle valve, ball valve or other valve that is selected for its volume and fluid flow properties. The first valve 103 and second valve 104 may feature a small dead volume and precise open and close control. The first valve 103 and second valve 104 may controlled to allow or prevent a specific fluid flow to the phase transition cell 106 and/or to allow backflushing of the filter 105. The first valve 103 and second valve 104 may be closed completely in some operations. The first valve 103 and second valve 104 may be selected to be modular and low cost for maintenance and repair.

The control of the pressure within the apparatus 100 may use a pressure control device such as a sapphire based piston 114. The control of the pressure in the apparatus 100 is adjusted by moving the piston 114 to change the volume within the piston housing and, thus, the sample volume. The apparatus' 100 small dead volume (less than 0.5 mL) facilitates pressure control and sample exchange. In some embodiments, the depressurization or pressurization rate of the fluid may be less than about 200 psi/second. In some embodiments, the fluid may be circulated through the apparatus 100 at a volumetric rate of no more than 1 ml/sec. Teflon, sapphire, alumina, ceramic, zirconia, or metal with seals may be selected for some components for various embodiments of the pressure control device. Within the piston 114, smooth hard surfaces may be used to minimize friction of the moving piston and both energized and dynamic seals may be used.

The densitometer 109 and viscometer 108 are configured for use in the system. The vibrating tube densitometer 109 measures the resonant frequency of a thin-walled tube of volume 20 microliters driven to oscillate using the Lorentz force. By prior calibration over the relevant pressure and temperature range the density of the fluid that is circulated through the tube may be deduced. In principal, the fractional frequency shift experienced by the resonator is not scale dependent meaning that the measurement volume can be even further reduced, though the resonance amplitude would be reduced as the cross-sectional area of the tube's path is reduced. For viscosity measurements, the vibrating wire viscometer 108 operates by measuring the decrement (inverse of twice the quality factor) of a resonating wire immersed in the fluid. Interpretation is provided by using the methods described in Retsina, Richardson, Waketam, “The theory of a vibrating rod viscometer,” Applied Scientific Research, 43:325-46 (1987), which is incorporated by reference herein. These sensors perform with a volume of no more than 20 microliters and operate at elevated temperature and pressure.

In order to benchmark the measurement of saturation pressure with this system, synthetically prepared live fluids were created by starting with known quantities of liquid n-alkanes (for example n-pentane, n-hexane, and n-heptane) determined gravimetrically. The alkanes were placed in a sample bottle of known volume and pressurized to approximately 1800 psi with partial pressures of methane and ethane. The two-phase sample was isolated with a valve, pressurized to 10,000 psi, and rocked overnight so as to completely dissolve the methane and ethane into the liquid phase. This produced a sample of known composition, but with unknown saturation properties (Table 2). Based on the known composition, equation of state models can be used to predict the subsequent saturation pressure as a function of pressure and temperature. In practice, however, the disparate critical points of the individual components and insufficiently developed mixing rules for these mixtures made such prediction useful for qualitative prediction. However, measurements with a conventional view cell allowed us to determine the phase envelope with great accuracy and these data will be used to benchmark our mini PVT system.

A conventional phase detection view cell was used to validate measurements obtained with the mini PVT system. This system includes two sample chambers with volumes of approximately 20 mL each. A magnetically coupled stirrer was used to agitate the fluid during depressurization. This agitation allowed the fluid to overcome the nucleation barrier of the phase transition. The two sample chambers were connected in series and an optical view cell, installed between the chambers, was used to monitor any phase change during depressurization. In addition, the pressure was monitored with a quartzdyne pressure gauge as the volume of the system was slowly increased, allowing us to confirm the optically detected phase transition by subtle shifts in the P-V (pressure-volume) curve.

An example of the simultaneous measurements undertaken during depressurization of a single-phase live fluid may be described as follows. At the beginning of the experiment, the system is charged with the fluid to be measured. The volume between the SSB bottles is initially occupied with a pressurized fluid from a previous experiment and prompts flushing. The pressure in the sample SSB is elevated to be about 150 psi higher than that in the waste SSB and valves are opened to allow the sample to flow through the PVT sensors and into the waste SSB. Note that both pressures are chosen to be several thousand psi above the bubble point of the sample. After pumping a volume of sample fluid that is roughly about 5-10 times that of the mini PVT system volume, the HIP AF1 valves (V1 and V2) are closed and the measurements commence. The pressure in the isolated portion of the flowline is decreased by decreasing the pressure on the hydraulic side of the microreactor piston with an Isco pump.

The optical intensity of the phase transition cell is monitored during the depressurization stage. In this example, the system has been charged with a live oil such that depressurization results in the production of bubbles. The bubble point is easily detected when the optical density increases. Two examples are shown in FIG. 16 where the optical density can be seen to increase suddenly at approximately 3940 psi when thermal nucleation is applied, but at 3800 psi when not applied. The former is very close to that measured by a conventional view cell. FIG. 16A is an example of the optical signal during depressurization when thermal nucleation was applied and not applied. The increase in optical density due to the presence of bubbles occurs at the thermodynamic saturation pressure (3940 psi, indicated by dashed line) when thermal nucleation is applied. Without thermal nucleation bubbles do not emerge until a substantially lower pressure (3800 psi).

During depressurization the density and viscosity are simultaneously recorded and the results are presented in FIG. 16B. This data set was obtained simultaneously with that of FIG. 16A. Focusing first on the density data, the density decreases slowly as the pressure is decreased from 5000 psi. This density decrease is seen in a fluid with properties similar to that of a black oil and the magnitude of the compressibility will be discussed further in a subsequent section. At 3800 psi the density rapidly decreases and becomes less stable as the sample has split into a gas and a liquid phase in the densitometer. While it is well-known that the liquid phase becomes denser for pressures below that of the bubble point, the densitometer is measuring an average density of both the liquid and gas phases and shows a decrease for pressures below the bubble point. Once a second phase has formed in the entire system, the total compressibility increases dramatically and this can be detected directly in the behavior of the densitometer. This transition occurs at a pressure far below the thermodynamic saturation pressure since no thermal nucleation in the densitometer.

A similar trend can be observed with the viscosity. The viscosity of the single phase sample decreases as the pressure is decreased until gas begins to emerge at 3800 psi. Again, this bubble point pressure is far below that detected in the phase transition cell since there is no thermal nucleation in the vibrating wire viscometer. At this point the viscosity of the remaining liquid increases, as is seen on measurements of the viscosity of the liquid phase below that of the saturation pressure. It can be speculated that the fluid in the viscometer, like that of the densitometer, has a very low volume fraction of bubbles unevenly distributed in the fluid. This increase in measured viscosity therefore indicates that the viscometer wire is more sensitive to the liquid than the gas phase in this case. Note that the density of the fluid measured by the vibrating tube densitometer is employed for calculating the viscosity.

The phase diagram of the multi-alkane sample was measured with the mini PVT apparatus over a temperature range from 25° C. to 125° C. using the techniques described above. The single and multi-phase regions are labeled accordingly on FIG. 17. FIG. 17 provides a phase diagram measured with the multi alkane sample. The measurements have been plotted with the mini PVT cell with and without thermal nucleation, respectively, measurements with nucleation, are in good agreement with those measured with the conventional PVT view cell.

The phase envelope follows the curve one would expect for a light oil and is rough agreement with the traditional PVT simulations, but the cricondenbar of the measurements is 400 psi lower than that of the simulations, illustrating that such simulators should be used with an appreciation of their limitations. The saturation pressures measured by the conventional view cell approach agree very well with the saturation pressures measured by the mini PVT system when thermal nucleation is applied. The measurements without thermal nucleation are consistently lower than those with thermal nucleation for temperatures lower than that of the cricondenbar. For temperatures above 100° C., the difference between these two measurements becomes minimal. The difference between the saturation pressures measured with and without thermal nucleation is plotted in FIG. 18A and is labeled “Supersaturation(psi)” because it is shown in pressure and is indicative of the nucleation barrier which exists for initial bubble formation in oil samples. This measurement is indicative of the nucleation behavior of the fluid when charged into the mini PVT system and does not represent a fundamental property of the fluid. It was generally observed that the supersaturation decreased as the temperature was increased. The nucleation barrier in condensate samples was found to be minimal.

FIG. 18B is a plot of density as a function of pressure for low (23° C.) and high (125° C.) temperatures. Arrows indicate approximate positions of saturation pressures. At the low temperature, a distinct kink can be seen in the density plot, but at the high temperature no kink is discernible. This illustrates why a phase transition cell facilitates determination of the saturation pressure, especially for samples beyond the critical point.

While a small kink can be seen in the density and viscosity's pressure dependence in FIG. 16B, the phase transition cell may provide the most certain detection of the saturation pressure since this is where thermal nucleation takes place. In FIG. 18B, the pressure dependence of two fluid densities is shown; one below the critical point and one above, for the multi-alkane sample. At the lowest temperature (below its critical point), the density experiences a detectable kink near the phase boundary, as indicated by the arrow. While this pressure does not correspond to the thermodynamic saturation pressure, it does show that density can be used as an indicator of a phase change for a fluid well below its critical point. However, for the highest temperature, where the fluid is above the critical temperature, the density smoothly decreases with pressure with no indication of the phase change.

A live oil was obtained downhole with a formation evaluation tester in order to further test the mini PVT system with a real crude sample. The fluid was maintained at elevated pressure during transport at ambient temperature and was homogenized at formation temperature by rocking for one week. The saturation pressure was measured from about 22.7° C. to about 148.7° C. and the optical densities as a function of pressure are plotted for each temperature in FIG. 19A. The optical intensities have been shifted for clarity and thermal nucleation was applied during each of these measurements. The saturation pressure for each temperature can easily be determined by the deviation of each line from horizontal. The resulting phase diagram is plotted in FIG. 19B, including data obtained with and without thermal nucleation. On average the saturation pressures measured with thermal nucleation are 200 psi higher than those measured without thermal nucleation. In this case the measurements with the conventional view cell were limited to the reservoir temperature, but good agreement may be seen between the conventional view cell with the saturation pressure measured with thermal nucleation. Additional information is available from U.S. patent application Ser. No. 13/800,896, filed on Mar. 13, 2013 which is incorporated by reference herein.

The density for each depressurization temperature is plotted in FIG. 20A as well as the corresponding viscosity (FIG. 20B). The high precision of both the vibrating tube densitometer and the vibrating wire viscometer allows us to resolve small shifts in the density and viscosity during depressurization. The sharp kink in the curve for each temperature may indicate that the sample has broken phase. Note that this apparent saturation pressure does not correspond to the thermodynamic saturation pressure since there is no agitation in either the densitometer nor in the viscometer to overcome the nucleation barrier. For pressures below this kink, the measured density drops rather rapidly with pressure and the measured viscosity increases rather rapidly, consistent with the phase diagram presented in FIG. 19B. In this case, the pressure dependence of the viscosity appears similar to those found in standard textbooks concerning the viscosity of the liquid phase of a live oil about the saturation pressure.

The high precision of the densitometer enables calculation the compressibility for each individual temperature (FIG. 21). The behavior of the compressibility is for that of a black oil, including the decreased compressibility at high pressure and the increased compressibility at higher temperature. The waviness seen in the data at 95 C is an artifact of the densitometer interpretation and should not be interpreted as a property of the fluid.

The operation of a mini PVT apparatus may occur with a total internal volume of approximately 500 microliters. Some embodiments may have an internal volume of 300 microliters, 100 microliters, 50 microliters, 30 microliters or 10 microliters. This apparatus is able to operate at pressure and temperatures consistent with downhole requirements and exploits novel sensors such as a microfluidic densitometer, a microfluidic viscometer, and a phase transition cell that uses thermal nucleation. The compatibility with true oilfield crude oils and measured a phase diagram that is consistent with that measured with a conventional view cell that use a comparatively large volume of fluid.

Although a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from embodiments disclosed herein. Accordingly, all such modifications are intended to be included within the scope of this disclosure. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. 

We claim:
 1. An apparatus for characterizing a fluid, comprising: a phase transition cell to receive the fluid; a piston to control pressure of the fluid; a pressure gauge to measure the pressure of the fluid and to provide information to control the piston; at least one measurement sensor to measure a property of the fluid; and connectors to fluidly connect the cell, piston, and pressure gauge, wherein the apparatus utilizes a total fluid volume of about 1.0 mL or less.
 2. The apparatus of claim 1, further comprising a membrane.
 3. The apparatus of claim 2, wherein the membrane prevents particles with a dimension of 10 microns or greater to flow through the membrane.
 4. The apparatus of claim 2, wherein the measurement sensor is selected from the group consisting of a densitometer and a viscometer.
 5. The apparatus of claim 1, wherein the at least one measurement sensor comprises a densitometer.
 6. The apparatus of claim 1, wherein the at least one measurement sensor comprises a viscometer.
 7. The apparatus of claim 1, further comprising a membrane, a densitometer, a viscometer, an inlet valve, and an exit valve.
 8. The apparatus of claim 7, wherein the fluid flows through the membrane, then the inlet valve, then the phase transition cell, then the densitometer, then the viscometer, then the pressure gauge, then the piston, then the exit valve.
 9. The apparatus of claim 1, wherein the fluid has a total fluid volume of about 0.5 mL or less.
 10. The apparatus of claim 1, wherein the connectors electrically isolate when forming a fluidic connection.
 11. The apparatus of claim 1, wherein the connectors have a total internal volume of less than 1 mL.
 12. The apparatus of claim 1, wherein the connectors comprises tubing.
 13. The apparatus of claim 12, wherein the tubing has an internal diameter of about 1.0 mm or less.
 14. The apparatus of claim 13, wherein ends of the tubing comprise isolation components.
 15. The apparatus of claim 1, wherein the cell, piston, and pressure gauge have a fluid volume less than about 2 ml.
 16. The apparatus of claim 1, wherein the pressure gauge has a dead volume of less than 0.5 ml.
 17. The apparatus of claim 1, wherein the volume of the apparatus is configured to fit in a wellbore.
 18. The apparatus of claim 24, wherein the volume is contained in a cylindrical shaped housing with an inner diameter of 3.875 inches or less.
 19. The apparatus of claim 1, further comprising a Peltier cooler.
 20. A method to characterize a fluid, comprising: observing a fluid in a phase transition cell; measuring a pressure of the fluid in the cell; and adjusting a pressure control device in response to the measuring, wherein the external volume of the phase transition cell, piston, and gauge is less than 10 liters.
 21. The method of claim 20, wherein the depressurization or pressurization rate of the fluid is less than 200 psi/second.
 22. The method of claim 20, wherein the fluid is circulated through the system at a volumetric rate of no more than 1 ml/sec.
 23. The method of claim 20, further comprising measuring a temperature.
 24. The method of claim 20, further comprising adjusting the temperature.
 25. The method of claim 24, wherein adjusting the temperature comprises increasing the fluid flow through the cell.
 26. The method of claim 25, wherein adjusting the temperature comprises a Peltier cooler. 